Although especially applicable to “The Input/Loss Method” as installed at coal-fired power plants, this invention may be applied to any on-line monitoring method and any other of the “Input/Loss methods”, installed at any thermal system burning a fossil fuel. Such monitoring is assumed to be conducted in a continuous manner (i.e., on-line), processing one monitoring cycle after another, each cycle includes determining stoichiometric balances of the combustion process and, specifically, the fuel's heating value. Specifically, The Input/Loss Method and its associated technologies are described in the following United States patent applications and resulting patents: Ser. No. 09/273,711 which issued on Feb. 18, 2003 as U.S. Pat. No. 6,522,994 (hereinafter termed '994): Ser. No. 09/630,853 which issued on Jun. 24, 2003 as U.S. Pat. No. 6,584,429 (hereinafter termed '429) and Ser. No. 10/087,879 filed on Mar. 1, 2002 which issued on Mar. 30, 2004 as U.S. Pat. No. 6,714,877 (hereinafter termed '877). One of the Input/Loss methods, a rudimentary method, is described in U.S. Pat. No. 5,367,470 issued Nov. 22, 1994 (hereinafter termed '470), and in U.S. Pat. No. 5,790,420 issued Aug. 4, 1998 (hereinafter termed '420).
Most fossil-fired thermal systems are controlled through Distributed Control. Systems (termed DCS). There are two major functional control operations within the DCS structure associated with a power plant: the control of the steam generator (the boiler), and the control of the turbine cycle. Control of the steam generator is accomplished through the so-called Boiler Master, a designated portion of the DCS. Control of the turbine cycle (and specially the steam turbine proper) is accomplished through the so-called Turbine Master, a designated portion of the DCS.
Typically the DCS may control the system using one of three modes, selected by the system operator: a Boiler Follow Mode, a Turbine Follow Mode, or a Fully Coordinated Mode. In a Boiler Follow Mode the boiler is slave to the load demands on the system by controlling the fuel firing rate such that the turbine throttle pressure is held to a desired set point. In a Turbine Follow Mode the turbine's throttle values are adjusted to control throttle pressure (i.e., the inlet pressure to the steam turbine). Typically, a power plant employs the Boiler Follow Mode. Turbine Follow Mode may be employed when the system is reducing load, or when a highly stabilized boiler operation is desired; turbine output will vary. When using the Fully Coordinated Mode the fuel firing rate is adjusted to control throttle pressure while, at the same time, the throttle valves are adjusted to control useful output from the system. As is typical with most DCS control logic, the absolute values of system parameters, such as fuel flow, combustion air flow throttle pressure and the like, are not required; only their relative, sealed, values. High consistency in such signals is, not only desired, but required for stable operation. For example, if the system indication of fuel flow is inconsistent, producing discontinuities in the signal and/or producing opposing trends versus the actual, unstable operations with dangerous results to equipment and personnel may be expected.
In addition to the requirement of obtaining consistent signals used by the DCS, when the system operator changes control modes, the DCS must response in a seamless manner. This is to say, for example, that when changing from a Boiler Follow Mode to a Turbine Follow Mode, the operation of the power plant should not become unstable. In cases when any instability exceeds design limits, the system will trip off-line, causing loss of all useful output. If the system is tripped from a high load, this most likely will cause considerable stress on materials and shorten fatigue life of equipment. Industrial art has developed a technique whereby changes in control modes may be made more seamless through the use of a numerical input to the DCS termed a “Btu-Compensator”. Typically the Btu-Compensator value is the heating value of the fuel. Use of the Btu-Compensator is intended to maintain a constant output from the Boiler Master such that the Boiler Master may trim controls of the system from a point which is common to all operational modes; modes of Boiler Follow, Turbine Follow and Fully Coordinated. Such a common point used for trim control implies seamless transitions between modes. Typically, the Btu-Compensator value is either input by the operator, or, and most commonly, is left as a constant value. If input by the operator, the Btu-Compensator value is based on his/her guess, laboratory analysis of the fuel (which may be many days old), the control engineer's judgement, or from an on-line instrument analyzer of the fossil fuel. Use of an on-line instrument analyzer may have functionality in supplying the Btu-Compensator, but in general these instruments are not reliable and thus have not received acceptance by the power industry.
The industrial art of controlling power plants and fossil combustion is well established. It relies on direct instrumentation of process streams; for example, the flow of fuel (e.g., coal flow), the flow of combustion air, pressure measurements, the sensing of valve positions, etc. In coal-fired power plants, fuel flow is commonly measured using scales placed on conveyer belts. Combustion air flow is measured using pitot tubes. Throttle pressure is measured using pressure sensors. Such measurement techniques are considered common art, having been established over 120 years of producing commercial electrical power. However, needed by the power industry are methods which improve DCS performance and control of fossil-fired thermal systems, such as power plants or steam generators, and specifically improvements to the consistency of base signals employed by the DCS and improvements in DCS stability when transitioning between operational modes.
The patents '470 and '420 make no mention of the use of parameters which may be used in power plant control systems. Although the technologies of '994 and '429 support this invention, they make no mention of parameters which may be used in control systems. The technology of '877 supports and adds to the technologies of '994 and '429 and thus supports this invention. Although the methods of '994 and '429 are highly useful, the present invention further improves these methods as applied to power plant control systems and systems burning fossil fuels. There is no known art related to this invention.